Re: measuring system wide CPU usage ignoring idle process

[Jiri Olsa <jolsa@redhat.com>]

On Mon, Nov 20, 2017 at 09:24:42PM +0100, Milian Wolff wrote:
> On Montag, 20. November 2017 15:29:08 CET Jiri Olsa wrote:
> > On Mon, Nov 20, 2017 at 03:00:46PM +0100, Milian Wolff wrote:
> > > Hey all,
> > > 
> > > colleagues of mine just brought this inconvenient perf stat behavior to my
> > > attention:
> > > 
> > > $ perf stat -a -e cpu-clock,task-clock,cycles,instructions sleep 1
> > > 
> > >  Performance counter stats for 'system wide':
> > >        4004.501439      cpu-clock (msec)          #    4.000 CPUs utilized
> > >        4004.526474      task-clock (msec)         #    4.000 CPUs utilized
> > >        945,906,029      cycles                    #    0.236 GHz
> > >        461,861,241      instructions              #    0.49  insn per
> > >        cycle
> > >        
> > >        1.001247082 seconds time elapsed
> > > 
> > > This shows that cpu-clock and task-clock are incremented also for the idle
> > > processes. Is there some trick to exclude that time, such that the CPU
> > > utilization drops below 100% when doing `perf stat -a`?
> > 
> > I dont think it's the idle process you see, I think it's the managing
> > overhead before the 'sleep 1' task goes actualy to sleep
> > 
> > there's some user space code before it gets into the sleep syscall,
> > and there's some possible kernel scheduling/syscall/irq code with
> > events already enabled and counting
> 
> Sorry for being unclear: I was talking about the task-clock and cpu-clock 
> values which you omitted from your measurements below. My example also shows 
> that the counts for cycles and instructions are fine. But the cpu-clock and 
> task-clock are useless as they always sum up to essentially `$nproc*$runtime`. 
> What I'm hoping for are fractional values for the "N CPUs utilized".

ugh my bad.. anyway by using -a you create cpu counters
which never unschedule, so those times will be same
as the 'sleep 1' run length

but not sure now how to get the real utilization.. will check

jirka